The present invention relates to a method of acquiring granular growth of a microorganism in a reactor containing a liquid phase which comprises a substrate, wherein in a first step said substrate is converted by the microorganism resulting in the formation of and growth on a phase comprising the organism while the liquid phase is being mixed, in a second step mixing in the reactor is stopped to allow part of the solid phase to settle, and in a third step the reactor is partly emptied by discharging the top part of the reactor contents, which reactor is subsequently replenished with substrate-comprising liquid, to repeat steps 1 to 3.
Such a method has been described by Sung S. et al. (Laboratory studies of the anaerobic sequencing batch reactor, in Water Environment Research, 67 (3), p. 294, 1995). In this method an anaerobic conversion is carried out, the contents of the reactor are then clarified in characteristically 10-30 minutes after which the top part of the reactor contents is discharged. According to this publication work with anaerobically activated sludge has long been known, although it was not recognized in the beginning (1966) that what was occurring was xe2x80x9cgranulationxe2x80x9d of the biomass. Granulation under methanogenic conditions (that is to say in the absence of oxygen) is often explained by the specific need of the respective organisms to exchange substrates the so-called xe2x80x9cinterspecies hydrogen transferxe2x80x9d, or to reduce the toxicity of oxygen.
Surprisingly, applicant has found that granulation can also occur under aerobic and turbulent conditions.
The method according to the invention is therefore characterized in that the microorganism is an aerobic microorganism, in that at least during the first step a third phase is present, which third phase comprises oxygen-containing gas being fed to the reactor during the first step while the contents of the reactor are kept in turbulence, and in that settling occurs in the second step and takes less time than the height of liquid in the reactor at the end of the first step divided by a settling velocity of at least 5 meters per hour.
The formation of granules under turbulent aerobic conditions is unexpected, because organisms in a granule are under very great stress as due to the size of the granule, nutrients have to diffuse over large distances before reaching the interior of a granule. The fact that in addition aerobic organisms require oxygen, increases the stress still more so that one would expect such aerobic organisms only to flocculate. In addition, the expert would expect the granules to disintegrate due to the great shearing forces caused by the turbulence.
In the present application an aerobic microorganism is understood to be both an obligate and a facultative aerobic microorganism.
Preferably the compound is fed to the reactor in pulses. This causes the organisms in a granule to be flooded with compound. As the organisms at the outside of the granule are unable to process such a supply of compound, the compound gets the opportunity to diffuse into the interior of the granule. This is especially important if the compound to be converted is a nutrient. An example of this is carbohydrate fermentation for the preparation of lactic acid.
In accordance with a favourable embodiment the contents of the reactor are substantially continuously mixed during the first step.
To promote the formation of aggregates, Sung et al. prefer periodical mixing during which only slight shearing forces occur. According to the invention, however, continuous turbulent mixing subjects flocs to mixing forces, allowing them to be discharged more easily in the third step. In addition, applicant has established by experimentation that if microorganisms are subjected to great shearing forces, the result is a more robust granule. Thus according to the invention the organisms in the reactor form into granules more quickly.
Turbulent mixing is conveniently carried out by feeding oxygen-containing gas into, for instance, an airlift-reactor or bubbling-bed reactor.
An interesting application of the method according to the invention is characterized in that the conversion is a nitrification-denitrification-conversion in which the oxygen-containing gas is only supplied during the first part of the first step for the completion of the nitrification, and that the reactor operates during the remainder of the first step under substantially anaerobic conditions for the completion of the denitrification. If desired, the gas may during this first step be recirculated over the reactor. Due to recirculation all oxygen is used up and turbulence is maintained.
When applying the method according to the invention, the organism-comprising granules must be present, or at least conditions promoting the formation of granules must be provided before starting up the reactor. It is useful, for instance, to feed the reactor with carrier particles to which organisms adhere, or are able to adhere. It has been shown that a mycelium-forming fungus can also be used as carrier.
According to a very favourable embodiment settling occurs in the second step, taking less time than the height of liquid in the reactor at the end of the first step divided by a settling velocity of at least 10 meters per hour, preferably at least 15 meters per hour. Thus the presence of granules in the reactor is strongly favoured in comparison with the presence of flocs. As mentioned above, Sung et al. describe a clarification step taking characteristically 10 to 30 minutes. The settling velocity is then only 1 meter per hour, and applied to aerobic organisms such a method will not result in granulation. While under anaerobic conditions the clarification step merely serves to separate organisms and treated water, is also the clarification step in the method according to the invention of essential importance for the induction of granulation.